At cell fate decision checkpoints, a precise series of events occur to establish developmentally appropriate gene expression profiles. At the molecular level, lineage-determinant transcription factors are required for the simultaneous activation and repression of genes that define the fate of a cell. Currently, it is unclear how these factors mechanistically achieve this precise control on a global level. In the immune system, naive CD4+ T helper cells begin with the potential to become a number of phenotypically distinct lineages, with key transcription factors committing them to a defined fate that is appropriate for a pathogenic insult. The T-box transcription factor T-bet is responsible for the differentiation of the Th1 cell lineage. Previous work has shown that T-bet positively regulates the effector cytokines and chemokine receptors that are the prototypic genes in Th1 cellular differentiation. To activate these select target genes, T-bet participates in at least three physically separable activities: 1) H3K27-demethylation, 2) H3K4-methylation, and 3) transactivation events that occur independent from the chromatin environment. It is currently unclear, however, whether these separable functional activities are required at all target promoters, or rather they are selectively utilized in a context-specific manner. In addition, the mechanism by which T-bet represses the gene expression profiles for the alternative helper T cell lineages and whether T-bet's ability to negatively regulate these genes requires the same or distinct activities is unknown. We will examine these questions on both a global and select target gene level. To this end, we will utilize mutant constructs deficient in each of T-bet's defined functional activities and assess their ability to regulate the global T-bet-dependent gene expression patterns in Th1 cell differentiation. We also will create mice deficient in defined interacting proteins to determine their biological relevance in regulating T-bet-dependent gene expression profiles in Type 1 immune responses. We will utilize this knowledge to model T-bet-dependent target gene networks that are based upon the mode of regulation. Together, these studies will allow us to define the mechanisms that are needed for the precise regulation of Th1 differentiation.